Substantially water-insoluble compounds useful in imaging are generally crystalline solids and are incorporated into imaging materials as dispersions or emulsions. In many cases, the compound useful in imaging is dissolved in one or more organic solvents, and the resulting oily liquid is then dispersed into an aqueous solution containing, optionally, dispersing aids such as surfactants and/or hydrophilic colloids such as gelatin. Dispersal of the oily liquid into the aqueous medium is accomplished using high shearing rates or high turbulence in devices such as colloid mills, ultrasonicators, or homogenizers.
In the art of dispersion making, the use of organic solvents has traditionally been considered necessary to achieve small particle sizes, to achieve stable dispersions, and to achieve the desired reactivity of the compound useful in imaging. Some compounds that might be useful in imaging cannot be dispersed in the above manner, however, because of their poor solubility in most organic solvents. In other cases, the compound of interest may have sufficient solubility in organic solvents, but it may be desirable to eliminate the use of the organic solvent to reduce the attendant adverse effects, for example, to reduce coated layer thickness, to reduce undesirable interactions of the organic solvent with other materials in the imaging element, to reduce risk of fire or operator exposure in manufacturing, or for environmental reasons or to improve the sharpness of the resulting image. These and other disadvantages can be overcome by the use of solid particle dispersions of the compound useful in imaging as described in UK Patent No. 1,570,362 to Langen et al, U.S. Pat. No. 4,006,025 to Swank et al, U.S. Pat. No. 4,294,916 to Postle et al, U.S. Pat. No. 4,294,917 to Postle et al, and U.S. Pat. No. 4,940,654 to Diehl et al.
Techniques for making solid particle dispersions are very different from the techniques used to make dispersions of oily liquids. Typically, solid particle dispersions are made by mixing the crystalline solid of interest with an aqueous solution that may contain one or more stabilizers or grinding aids. Particle size reduction is accomplished by subjecting the solid crystals in the slurry to repeated collisions with beads of hard, inorganic milling media, such as sand, spheres of silica, stainless steel, silicon carbide, glass, zirconium, zirconium oxide, alumina, titanium etc., which fracture the crystals. The bead sizes typically range from 0.25 to 3.0 mm in diameter. Ball mills, media mills, attritor mills, jet mills, vibratory mills, etc. are frequently used to accomplish particle size reduction. Milling techniques are described, for example, in U.S. Pat. Nos. 3,008,657 to Szegvari, 3,075,710 to Feld et al and 3,149,789 to Szegvari.
The stabilization of solid particle dispersions is much more difficult than the stabilization of conventional liquid droplet dispersions. Traditional ionic or nonionic alkyl or aryl stabilizers tend to stabilize liquid dispersions (emulsions) much better than solid particle dispersions. Conventional solid particle aqueous dispersions of photographically useful compounds are prepared using a dispersants such as a small molecule anionic surfactant or certain homopolymer surfactants in water. The dispersant, ideally should prevent flocculation of the dye particles upon milling down to the desired size (generally less than about 0.2 microns).